1. Field of the Invention
This invention generally relates to an insulated conduit having particular utility in subterranean wells and more particularly to a concentric walled insulated conduit having an annular space between the walls within which an insulating material is deposited and sealed therein.
2. Description of the Prior Art
In producing some subterranean wells, steam is injected into an injection well to increase recovery of hydrocarbons by reducing high viscosity crude oil, otherwise known as "heavy crude". The lower viscosity makes the oil more readily pumpable. One technique for doing this is to inject a high quantity of steam into the production zone containing "heavy crude" for an extended period of time, such as from about three to about five weeks. At that point, the viscosity of the heated crude will be reduced and will be readily pumpable through a production well in communication with the production zone. The injection well may also be modified for production. A steam "flood" may also be provided by known techniques, generally through an injection well, to drive the flood and the produced hydrocarbons into a nearby production well.
One of the major problems in injecting steam into a subterranean production zone through conventional well production tubing is that the steam loses a large quantity of its heat to the well bore casing and surrounding formation as it travels downwardly to the production zone. Attempts have been made in the past to reduce the heat loss of steam introduced into subterranean formations. One such attempt is disclosed in U.S. Pat. No. 3,511,282, issued on May 12, 1970. This patent discloses a dual-wall tube structure having insulation sealed in the annulus between the inner and outer walls by bushings respectively welded at each end between the inner and outer walls by bushings respectively welded at each end between the inner wall and the outer wall. The inner wall is prestressed in tension prior to being welded to the outer wall. The space defined between the inner and outer walls is filled with a conventional insulating material, such as calcium silicate. Although satisfactory in some oil field installations, large temperature differentials are encountered between the inner and outer walls, structural support must be provided between the inner and outer walls. In this case, even though the inner wall is prestressed in tension, the inner wall, as it is heated, will elongate with respect to the outer wall so that the inner wall may even change from a tension to a compression condition with the attendant danger of buckling. Centralizers are incorporated to reduce buckling but these centralizers introduce a heat loss path between the hotter inner tubular member and the colder outer tubular member. Centralizers have been fabricated using cordierite in an annular ring configuration and in a segmented configuration in which axial circumferential discontinuous lengths segments are attached to the inner member to support the outer member.
Concentric walled thermal insulating conduit is also disclosed and claimed in U.S. patent application Ser. No. 264,728 entitled CONCENTRIC WALLED CONDUIT FOR TUBULAR CONDUIT STRING, and U.S. patent application Ser. No. 272,411 entitled INSULATING TUBULAR CONDUIT APPARATUS AND METHOD. The tubular conduits disclosed therein comprise concentric tubular members in which the inner tubular member has flared ends welded to the outer tubular member adjacent each end. Insulation is provided within the annular area between the two tubular members for reducing heat loss during steam injection. Insulating centralizer supports preferably formed of a material such as calcium silicate are employed to prevent buckling.